Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin (PAO) synthetic lubricants, primarily based on the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wide range of temperature,i.e.,improved viscosity index (VI), while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These new synthetic lubricants lower friction and hence increase mechanical efficiency across the full spectrum of mechanical loads from worm gears to steel gears and do so over a wider range of operating conditions than mineral oil lubricants.
Recently, novel lubricant compositions (referred to herein as HVI-PAO and the HVI-PAO process) comprising polyalpha-olefins and methods for their preparation employing as catalyst reduced chromium on a silica support have been disclosed in U.S. patent applications Ser. No. 210,434 and 210,435 filed Jun. 23, 1988, incorporated herein by reference in their entirety. The process comprises contacting C.sub.6 -C.sub.20 1-alkene feedstock with reduced valence state chromium oxide catalyst on porous silica support under oligomerizing conditions in an oligomerization zone whereby high viscosity, high VI liquid hydrocarbon lubricant is produced having branch ratios, i.e. CH.sub.3 /CH.sub.2, less than 0.19 and pour point below -15.degree. C. The process is distinctive in that little isomerization of the olefinic bond occurs compared to known oligomerization methods to produce polyalpha-olefins using Lewis acid catalyst. Lubricants produced by the process cover the full range of lubricant viscosities and exhibit a remarkably high viscosity index (VI) and low pour point even at high viscosity. The as-synthesized HVI-PAO oligomer has a preponderance of terminal olefinic unsaturation, or exo-olefinic groups.
In the preparation of the novel HVI-PAO lubricant, alpha-olefin dimer containing olefinic unsaturation can be separated from the oligomerization reaction. The composition of the dimer mixture conforms to the unique specificity of the oligomerization reaction in that little double bond isomerization is found. Separation of the dimer, representing non-lube range molecular weight material, is necessitated to control product volatility and viscosity. However, as oligomerization conditions are changed to produce the lower viscosity products of lower average molecular weight important to the marketplace, the non-lube range dimer fraction by-product yield increases in proportion to that lowering in average molecular weight of the oligomerization product. In the synthesis of low average molecular weight oligomers substantial amounts of products stop at the dimer stage. Once so formed, they are inert to the further incorporation of olefin in the HVI-PAO oligomerization reaction. The increase in dimer by-product yield represents a substantial economic burden on the overall process to produce useful lower viscosity lubricant in the HVI-PAO process.
Accordingly, it is an object of the present invention to provide a process for the conversion of alkenes containing exo-olefin groups to higher molecular weight lube range hydrocarbons.
It is another object of the present invention to provide a process for the utilization of the HVI-PAO process by-product dimer for the further production of synthetic lubricant fluid.
It is another object of the present invention to provide a process for the production of low viscosity lubricant exhibiting superior lubricant properties utilizing HVI-PAO dimer.
Yet another object of the instant invention is to provide novel lubricant compositions that display superior lubricant properties derived from alpha-olefin dimers.